In manufacturing paper from wood, the wood is first reduced to an intermediate stage in which the fibers in the wood are separated from their natural environment and transformed into a viscous liquid suspension called pulp. Of the various components of wood, the cellulose polymers are the most abundant and are the predominant molecules desired in the final pulp product. The second most abundant polymer, and least desirable pulp component, is lignin. Lignin is undesired because substantial amounts of lignin in pulp can deteriorate the smoothness of the final paper product and cause the paper to discolor when exposed to light. Lignin can also cause the pulp fibers to be rigid and weak.
Pulp may be produced from various types of woods using any one of several pulping techniques. The simplest of these techniques is the refiner mechanical pulping (RMP) method in which a mechanical milling operation grinds or abrades the wood in water until a desired state of freeness is achieved between its fibers. The RMP method is very efficient, typically converting approximately 95% of the dry weight of the wood into pulp. The RMP method, however, also leaves substantially all of the lignin in the pulp. As a result, RMP pulps generally provide low strength paper products having an opaque color. These paper products are generally used to manufacture newsprint or other low quality paper products.
Other pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), chemi-mechanical pulping (CMP), and the chemical pulping, sulfate (kraft) or sulfite processes. In the chemical based methods, a chemical/water solution is generally used to dissolve the lignin and promote the separation of the fibers. The absence of lignin, in turn, makes the final paper products stronger and less prone to discoloration. These products often include paper bags, shipping containers, printing and writing papers, and other products requiring strength.
In thermo-mechanical processes (e.g., TMP and CTMP), high temperatures are used to separate the fibers during refining. These processes generally require the refining to be carried out in one or more steps. The first step is usually a pressurized step with refining being performed at temperatures above 100° C. and immediately below or at the softening temperature of lignin. During this step, the pulp is typically mechanically processed using the RMP method. In subsequent steps, the pressure and temperature is usually modulated to achieve the desired state of freeness between the fibers.
Relatively high total electric energy amounts or high quantities of input wood are required to produce pulps using the above mentioned pulping techniques. In particular, high energy inputs are generally required to obtain fiber separation in woods rich in lignin as such woods typically call for extended refining periods and high temperatures and/or pressures. Recent studies have also suggested that even thermal or chemical softening treatments of such woods does not guarantee a lower total energy consumption. This is because unprocessed fibers which are only mildly separated by the thermal or chemical treatments are difficult to fibrillate during the refining mechanical process.
Fibrillation is necessary to increase the flexibility of the fibers and bring about the fine material characteristics of quality processed pulp. In fact, it has been suggested that a decrease in the energy consumption from an established level in various TMP and CTMP processes has been associated with the deterioration of certain pulp properties, including a reduction in the long fiber content of the pulp, a lower tear strength and tensile strength, and a higher shives content. (See U.S. Pat. No. 5,853,534, issued to Hoglund et al., Dec. 29, 1998). As a result, high energy consumption in TMP and CTMP processes has been generally necessary in today's pulping practices.
U.S. Pat. No. 5,853,534 describes a method for producing pulp which attempts to overcome the above described energy consumption problem by performing mechanical or chemi-mechanical pulp in at least two steps. In the disclosed process, wood material is fed into a first refining step where it is mechanically processed at a temperature less than the softening temperature of lignin, and then fed into a second refining step where it is mechanically processed at a temperature exceeding the softening temperature of lignin. This process purports to guide fractures and fracture indications into the wood's fiber walls not rich in lignin during the first step to allow the fiber material to be separated with low energy inputs in areas rich in lignin during the second step. The process also purports to release fine material from areas between the initial fracture zone and the middle lamina of the fiber material rich in lignin during temperatures above the softening temperature of lignin, thus also lowering the total energy consumption in the process.
What is needed is an alternative method for producing pulp in an energy efficient manner which also improves paper strength properties while decreasing pollution.